1. Field of the Invention
The present invention relates generally to a method and apparatus for coordinating the pacing of a heart and more particularly, to a method and apparatus for ventricular pacing that is triggered upon sensing an early ventricular event.
2. Description of the Related Art
Cardiac muscle needs to be electrically excited to depolarize causing a contraction. To depolarize, the muscle must reach a threshold voltage. Intrinsically, the threshold voltage is initiated by a nerve impulse. Once initiated, the depolarization wave propagates through the muscle causing the contraction. The depolarization can be recorded intracardially and/or extracardially. The recorded depolarization events are typically referred to as an electrocardiogram or ECG. An ECG recorded intracardially is more appropriately referred to as an electrogram. Typically, electrograms are recorded by electrodes placed endocardially in or epicardially on an atrium or ventricle. An ECG recorded extracardially is more appropriately referred to as a surface ECG. Surface ECGs are typically recorded from two or more electrodes placed at predetermined locations on a patient""s skin. A complete surface ECG recording typically utilizes a conventional twelve lead configuration.
The features in a surface ECG are typically labeled according to the electrical activity""s origin. The signal corresponding to the depolarization of the atria is called the P-wave. The signal corresponding to the depolarization of the ventricles is the QRS complex. The QRS complex can be described using three waves: the Q-wave; the R-wave; and the S-wave. The time interval from the P-wave to the R-wave is the PR interval. Thus, the PR interval is a measure of the delay between the electrical excitation in the atria and the ventricles.
Unlike surface ECG, electrograms mainly reflect local electrical depolarization. For example, an atrial electrogram mainly reflects the atrial depolarization. Therefore, an atrial electrogram corresponds to the P-wave in the surface ECG. Similarly, a ventricular electrogram mainly reflects ventricular depolarization, and thus, corresponds to a QRS complex of the surface ECG. However, it is quite often that the morphology of an electrogram may differ from its counterpart in a surface ECG, depending on the configuration of the recording electrode(s).
Currently, no consensus terminology describes the features of a ventricular electrogram. Borrowing terminology from surface ECGs, the largest peak in a ventricular electrogram is referred to as the R-wave, and the onset of the ventricular electrogram is referred to as the Q* point in the present disclosure. Physiologically, the Q* is considered the time of first or earliest detectable ventricular depolarization. Defined as the time of first detectable ventricular depolarization, the Q* concept can be applied to surface ECGs. Thus, the onset of the Q-wave in a surface ECG may be the first detectable ventricular depolarization coinciding with the Q* point of a ventricular electrogram. Thus, Q* may be measured from an electrogram or from a surface ECG.
Cardiac pacing has been used primarily to treat patients with bradycardia. A variety of pacing modes are used for the different syndromes of bradycardia. For example, for patients with normal atrial rhythm but slow ventricular rhythm due to 3rd degree AV node block, VDD mode is often the choice of therapy. In the VDD pacing mode, ventricular pacing is triggered, after an AV delay, by a sensed electrical event in the atrium. Thus, the heart rate is increased and the ventricular rate is maintained at the atrial rate.
Recently, there has been increasing interest in using electrical stimulation as an alternative therapy to treat congestive heart failure (CHF) patients who are refractory to conventional drug therapy. For example, VDD pacing has been applied to CHF patients with normal heart rate, but with abnormal ventricular conduction system. In these patients, electrical stimulation has been used to correct the electric activation pattern of the ventricle(s) rather than to maintain the heart rate as it does for bradycardia patients. In theory, stimulating at an otherwise delayed portion of the ventricle restores synchronous ventricular contraction and thus, improves hemodynamic performance. Therefore, VDD stimulation for CHF is frequently referred to as cardiac resynchronization therapy (CRT). Currently, CRT is mainly applied to the left ventricle (LV) or both ventricles (biventricular or BV) for CHF patients with bundle branch block (BBB).
However, a large number of CHF patients also have chronic atrial fibrillation (AF). For those patients, VDD mode cannot be applied because of unavailable and/or unreliable atrial sensing to trigger ventricular stimulation. Biventricular triggering (BVT) has been developed to allow treatment of patients suffering from AF. In BVT, bi-ventricular stimulation is triggered upon sensing a ventricular event in either ventricle. In theory, BVT may still provide some degree of coordinated ventricular contraction. However, BVT mode is less likely than other methods to provide highly synchronous ventricular contraction because of a time delay between ventricular depolarization and triggering. That is, by the time current methods sense a ventricular event, usually from a R-wave as seen in a ventricular electrogram, a large portion of the ventricle may have already been intrinsically excited through asynchronous slow muscle propagation due to the block of the fast conduction system. Thus, a need exists for an alternative triggering event that is early enough to trigger ventricular stimulation and allows for more reliable sensing in the AF patients.
The method and apparatus of present invention meet the above described needs and provide additional advantages and improvements that will be recognized by those skilled in the art upon review of this disclosure. The present invention provides an apparatus and method for ventricular pacing triggered by an early ventricular sensed event. This early event occurs earlier than the R-wave and exists whenever there is intrinsic ventricular depolarization regardless of atrial conduction. The pacing pulse is delivered immediately or following a short delay to either or both ventricles upon detection of such an early event.
In its broadest aspects, the present invention comprises an apparatus including a sensor that is configured to sense the depolarizations of the heart, the sensor feeding data to a processor that is programmed to identify an early ventricular electrical event and a pulse generator controlled by the processor and configured to provide a pacing stimulus to at least one ventricle of the heart based upon the occurrence of the event. Possible early ventricular electrical events include the onset (Q*) of ventricular depolarizations which can be detected from QRS complex, and the onset of HIS bundle depolarization, which can be detected from a HIS bundle electrogram.